September, 2021 Int J Agric & Biol Eng Open Access at https://www.ijabe.org Vol. 14 No. 5 255 Optimizing the drying parameters of a fixed bed with reversing ventilation for peanut using computer simulation Jianchun Yan, Huanxiong Xie * , Hai Wei, Huichang Wu, Zhaoyan You (Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China) Abstract: To obtain the optimal operation parameters of fixed-bed reversing ventilation drying of peanuts, a set of partial differential equations indicating the heat and mass transfer relationships between the peanut pods and air during drying was proposed. Then, a series of discretized models were established for simulation, and the time consumed, unevenness, and energy consumption for batch drying were calculated. The results showed that reversing ventilation and segmented drying was helpful to these issues for high drying ability. The optimal operation parameters were determined by uniform design experimentation of mathematical simulation. The result showed that when the moisture content (wet basis) was above 22%, a ventilation velocity of 0.46 m/s was optimal; when the moisture content was between 8% and 22%, a ventilation velocity of 0.20 m/s was optimal. Using the optimal parameters, the computer simulating result was compared with the experimental results. The correlation coefficients between the simulating and the experimental values for the temperature and moisture content were all above 0.98 and the quality of dried peanuts was close to that of natural sun-dried ones, which indicates that the optimization results of the drying parameters are highly reliable. Keywords: peanut, fix bed drying, reversing ventilation, simulation, optimization DOI: 10.25165/j.ijabe.20211405.6354 Citation: Yan J C, Xie H X, Wei H, Wu H C, You Z Y. Optimizing the drying parameters of a fixed bed with reversing ventilation for peanut using computer simulation. Int J Agric & Biol Eng, 2021; 14(5): 255–266. 1 Introduction Peanut is the dominant high-quality oil-bearing crop and an important food protein resource in China. For the past decade, China has been the largest peanut producer and has the second-largest planting area [1] . Influenced by agricultural policy, population mobility, and land transfer, the mechanized harvesting of peanuts has increased over years; consequently, harvesting is synchronized, and fresh peanut pods can be quickly amassed and large amounts of peanuts wait for drying [2] . This rapid expansion means that the drying facilities and labor available in rural areas have not satisfied the demand for drying [2,3] . On overcast and rainy days, mildew and rot of the crop usually occur, and aflatoxin contamination is a relatively serious concern [4,5] . Economic and practical peanut-drying methods are required. Some developed countries with large-scale peanut planting, such as the United States, have developed mature peanut drying technology and equipment [6-8] . Peanut drying is usually divided into two stages: first, vines are dried in the field after digging; second, centralized drying is needed after pickup harvesting [9-12] . The entire drying process is intrinsically combined with digging, picking, storage, Received date: 2020-12-13 Accepted date: 2021-06-14 Biographies: Jianchun Yan, Assistant Professor, research interest: agricultural products drying and processing technology, Email: [email protected]; Hai Wei, Assistant Professor, research interest: agricultural products drying and processing technology, Email: [email protected]; Huichang Wu, Professor, research interest: agricultural products processing technology, Email: [email protected]; Zhaoyan You, Assistant Professor, research interest: agricultural products processing technology. *Corresponding author: Huanxiong Xie, Professor, research interest: agricultural products processing technology. Nanjing Research Institute for Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, No.100, Liuying, Xuanwu Disrict, Nanjing 210014, China. Tel: +86-25-84346256, Email: [email protected]. and processing. When the dryer is loading, the moisture content of peanuts (with their shells) is generally approximate to 20%. During the drying process, the hot air generated by burning liquefied petroleum passes through the material bed from the bottom to the top [13,14] . Based on the material bed thickness and initial moisture content, ventilation volumes are regulated to the recommended values [7] . Although the above-mentioned two-stage process has been widely used in the USA and Australia, in rural China, little time is allowed for the crop to be dried before the next crop is planted. To increase farm productivity, the drying of fresh peanut pods is a priority [3,15] . However, owing to the large moisture content reduction required, the drying effect of the above-mentioned method used in the USA is poor. In addition, high energy and equipment price has made the peanut drying expensive and promoting fresh peanut drying difficult in rural China. Generally, peanut farmers use the fixed-bed dryers as supplementary or emergency means of drying [2] . However, uneven drying is a problem. To improve the uniformity of fixed-bed drying, the fixed-bed reversing ventilation drying technique for peanuts has been proposed. However, heat waste during the middle and later stages of drying is a serious issue. It is assumed for batch drying that the more uniform the drying is, the more the air volume should be provided and the more energy is consumed. Numerical simulation has been widely used to describe the drying process and develop production guidelines for peanuts and other crops. Parti and Young [16] established a peanut bulk drying model (PEADRY8), which was compared with the experimental results for Virginia-type peanuts. The model took the peanut pod as two separate components with moisture movement in both liquid and vapor forms. Chai and Young [17] studied the effects of airflow rate on drying times and costs of conventional and recirculating peanut drying facilities using numerical simulation. Yang et al. [18] studied the simulation of peanut drying in a
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September, 2021 Int J Agric & Biol Eng Open Access at https://www.ijabe.org Vol. 14 No. 5 255
Optimizing the drying parameters of a fixed bed with reversing ventilation
for peanut using computer simulation
Jianchun Yan, Huanxiong Xie*
, Hai Wei, Huichang Wu, Zhaoyan You (Nanjing Institute of Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China)
Abstract: To obtain the optimal operation parameters of fixed-bed reversing ventilation drying of peanuts, a set of partial
differential equations indicating the heat and mass transfer relationships between the peanut pods and air during drying was
proposed. Then, a series of discretized models were established for simulation, and the time consumed, unevenness, and
energy consumption for batch drying were calculated. The results showed that reversing ventilation and segmented drying
was helpful to these issues for high drying ability. The optimal operation parameters were determined by uniform design
experimentation of mathematical simulation. The result showed that when the moisture content (wet basis) was above 22%, a
ventilation velocity of 0.46 m/s was optimal; when the moisture content was between 8% and 22%, a ventilation velocity of
0.20 m/s was optimal. Using the optimal parameters, the computer simulating result was compared with the experimental
results. The correlation coefficients between the simulating and the experimental values for the temperature and moisture
content were all above 0.98 and the quality of dried peanuts was close to that of natural sun-dried ones, which indicates that the
optimization results of the drying parameters are highly reliable.
Keywords: peanut, fix bed drying, reversing ventilation, simulation, optimization
DOI: 10.25165/j.ijabe.20211405.6354
Citation: Yan J C, Xie H X, Wei H, Wu H C, You Z Y. Optimizing the drying parameters of a fixed bed with reversing
ventilation for peanut using computer simulation. Int J Agric & Biol Eng, 2021; 14(5): 255–266.
1 Introduction
Peanut is the dominant high-quality oil-bearing crop and an
important food protein resource in China. For the past decade,
China has been the largest peanut producer and has the
second-largest planting area[1]. Influenced by agricultural policy,
population mobility, and land transfer, the mechanized harvesting
of peanuts has increased over years; consequently, harvesting is
synchronized, and fresh peanut pods can be quickly amassed and
large amounts of peanuts wait for drying[2]. This rapid expansion
means that the drying facilities and labor available in rural areas
have not satisfied the demand for drying[2,3]. On overcast and
rainy days, mildew and rot of the crop usually occur, and aflatoxin
contamination is a relatively serious concern[4,5]. Economic and
practical peanut-drying methods are required. Some developed
countries with large-scale peanut planting, such as the United
States, have developed mature peanut drying technology and
equipment[6-8]. Peanut drying is usually divided into two stages:
first, vines are dried in the field after digging; second, centralized
drying is needed after pickup harvesting[9-12]. The entire drying
process is intrinsically combined with digging, picking, storage,
Received date: 2020-12-13 Accepted date: 2021-06-14
Biographies: Jianchun Yan, Assistant Professor, research interest: agricultural
products drying and processing technology, Email: [email protected]; Hai Wei,
Assistant Professor, research interest: agricultural products drying and
processing technology, Email: [email protected]; Huichang Wu, Professor, research interest: agricultural products processing technology, Email:
[email protected]; Zhaoyan You, Assistant Professor, research interest:
agricultural products processing technology.
*Corresponding author: Huanxiong Xie, Professor, research interest:
agricultural products processing technology. Nanjing Research Institute for
Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, No.100,
The equipment is comprised of a drying box, heat pump, fan,
ventilation devices, air uniform mechanism, and control system.
The temperature of hot air entering the drying box could be
adjusted to provide a constant temperature under an ambient
260 September, 2021 Int J Agric & Biol Eng Open Access at https://www.ijabe.org Vol. 134No. 5
temperature up to 20°C, with an accuracy of ±1°C. The
maximum ventilation capacity was approximately 24 000 m3/h, and
the fan motor speed was adjusted by the frequency converter, with
an accuracy of ±0.1 Hz. The material-loading area of the drying
chamber was 12 m2 (6 m×2 m), maximum loading thickness was
1 m, the distance between the loading punching plate and the floor
of the bottom air chamber was 450 mm, and minimum distance
between the top of the material layer and the cover was 450 mm.
Peanuts were harvested in Zhengyang County, Henan Province,
China, with an average moisture content of 40.8% (wet basis)
before drying. The average temperature and relative humidity of
ambient air were 24.1°C and 43.9%, respectively. The input
temperature of the drying air was set to 38°C, and the average air
velocity was regulated by a frequency converter based on the
experimental requirements.
2.5.2 Validation of simulation results
Peanut pods are characterized by large particle size and poor
fluidity; therefore, it is difficult to extract samples from the
material bed with a plug-in sampler to measure the moisture
content of the upper, middle, and lower layers of the material bed.
Therefore, a sampling cylinder was developed. The sampling
cylinder was a thin-walled cylindrical container with dense holes at
the bottom and wall (hole diameter, 5 mm; perforating ratio,
35.4%). The drying air entered and exited the container free from
the bottom and sidewalls. The diameter of the sampling cylinder
was 50 mm, and its height was 1000 mm.
To understand the changes in moisture content and temperature
in the upper, middle, and lower sections during drying, the volume
of peanuts was divided into 27 (3×9) test units in the horizontal
direction (Figure 4).
Figure 4 Schematic of testing area distribution
When loading materials, three sampling cylinders were placed
vertically at the center of each test unit, and the bottom of the
samplers was in contact with the porous bearing plate. As the
experiment continued, at 15 h, 30 h, and at the end of drying, one
sampling cylinder was removed from each test unit, and the
materials in the sampling cylinders were divided into three sections:
upper, middle, and lower section. The moisture contents of the
81 (3×27) testing points were then measured using the oven
method[40]. The moisture contents from the same height of the
layer materials were taken as a group, and the moisture contents of
the upper-, middle-, and lower-layer materials were calculated by
averaging each group.
Temperature sensors were placed at the center of each test unit
at the height of 165 mm, 500 mm, and 835 mm from the bearing
plate in the vertical direction. The number of temperature sensors
was 81 (3×27). The sensors of the same height of each test unit
were also taken as a group, and the average value of each group
was taken as the temperature value of the material in that layer.
The correlation coefficient was used to demonstrate the
statistical index of the relationship between simulating and
experimental values.
1
2 2
1 1
(Pr e Pr e)(Exp Exp)
(Pr e Pr e) (Exp Exp)
N
U U
U
N N
U U
U U
r
(29)
where, r is the correlation coefficient; PreU is the Uth simulation
prediction value; Pre is the mean of the predicted value; ExpU is
the Uth experimental value; and Exp is the mean of the
experimental value.
2.5.3 Quality tests of dried peanuts
When the drying process was completed, the dried peanuts
were discharged from their relevant ports and transported to a
designated area by the conveyor. In the process of peanut
discharge, a sample of peanuts was taken every 5 min. Five
samples were collected, and each sample had a mass of no less than
200 g. For every peanut sample, aflatoxin content was determined
based on ISO Standard 16050[41], the acid value was determined
based on ISO Standard 660[42], and the peroxide value was
determined based on ISO Standard 3960[43]. In addition, three
naturally dried peanut samples (total weight of samples was
approximately 3 kg, and the exposure time was approximately 5 d)
were selected as a control group, and the aflatoxin content, acid
value, and peroxide value of those samples were determined by the
same method. The flavor test was performed based on the ISO
Standard 6658[44].
3 Results and discussion
3.1 Numerical simulation
3.1.1 Peanut temperature and moisture content during drying
The temperature and moisture content (wet basis) of the
peanuts were simulated with an air velocity of 0.3 m/s, material bed
depth of 1 m, and airflow direction switching time of 3 h (Figure 5).
Owing to the ventilation sequence of the material layers changed
with air direction reversing each time, the temperatures of the
peanut layer at different heights gradually increased in a wave-like
shape with the increase in drying time. With the accumulation of
heat transferred from hot air to peanut layers and the reduction in
temperature difference between the material layers and hot air, the
fluctuation amplitudes gradually decreased as the temperature
approached the air input temperature. Because the bottom and top
materials were either the first or the last to contact hot air, the
temperature fluctuation amplitudes of the bottom and top materials
were the greatest. By contrast, the temperature fluctuation
amplitudes of the middle layers were small, because in the layers
near the middle position of the material bed, the ventilation
sequence changed mildly as the hot air direction changed. This
was similar to other results for airflow direction reversing
ventilation drying for carrots[25] and wheat[28].
In the initial stage of drying (0-3 h), air passed through the
material layer from the bottom to the top. As the air passed
through a critical position in the fixed bed, the relative humidity
was high sufficiently so that the moisture absorption balance was
achieved between the air and the layer material. Thus, the
moisture in the remaining material layers could not be absorbed.
When the ventilation direction was changed for the first time, the
upper material layers contacted hot air for the first time. The rate
September, 2021 Yan J C, et al. Optimizing the drying parameters of a fixed bed with reversing ventilation for peanut Vol. 14 No. 5 261
of moisture decreased in the top layer material accelerated, whereas
the moisture content of the bottom layer changed mildly. The
moisture difference of the whole bed thus decreased rapidly, and
this process was repeated until the average moisture content reached
8% wet basis. Compared with traditional fixed-bed drying[21,22,25,31],
the periodic change in the airflow direction effectively reduced the
drying unevenness in the ventilation direction[25,27,29], which
reduced the moisture content difference by 70%-85%.
a. Temperature of the entire bed b. Moisture content of the entire bed
c. Temperature of five positions d. Moisture content of five positions
Note: The ambient air temperature was 25°C and humidity was 50%; the material bed depth was 1 m; Hot air input temperature was 38°C, air velocity was 0.3 m/s, and
the airflow reversing time was 3 h.
Figure 5 Variation of peanut material bed temperature and moisture content during the drying process
3.1.2 Average temperatures and moisture contents at different
ventilation velocities
The average (volume-averaged) temperatures and moisture
contents of the entire bed at different ventilation velocities were
simulated when the air input temperature was 38°C, depth of the
material bed was 1 m, and airflow direction switching time was 3 h
(Figure 6). The average temperature of the entire bed generally
increased after the start of the drying process. It rose rapidly at
the start of the drying process and then gradually slowed down
until it approached the air input temperature. During this process,
the average temperature dropped suddenly; however, it rose rapidly
during every other period of reversal. Thus, the heat was quickly
withdrawn from the dryer through the air, resulting in a rapid
decrease in the temperature of the entire bed. Then the injection
of the “new” air led to a rapid increase and recovery of the average
temperature of the entire bed[2,26]. The lower the air velocity, the
greater the drop in the average temperature.
Correspondingly, the change in average moisture content can
be roughly divided into two stages. First, the decreasing rate of
the average moisture content was almost the same, and the
difference of the moisture content of the bed material increased and
then decreased rapidly. Here, the moisture in the peanut hulls
rapidly evaporated from the bed under air heating. Second, the
rate of decrease of the average moisture content gradually
decreased. As the ventilation direction changes with time, the
changes in moisture content difference showed a wave-like pattern
with a gradual decrease in amplitude. In addition, the lower the
air velocity, the greater the difference of the moisture content.
Here, the outward migration and evaporation of moisture of the
peanut kernels played a dominant role.
a. Average temperature b. Average moisture content c. Moisture content difference
Figure 6 Simulation results of the overall drying process with different ventilation velocities. The ambient air temperature was 25°C and
humidity was 50%; material bed depth was 1 m, hot air input temperature was 38°C, and airflow reversing time was 3 h
262 September, 2021 Int J Agric & Biol Eng Open Access at https://www.ijabe.org Vol. 134No. 5
3.1.3 Effects of air velocities and air direction switching time on
drying indices
Time consumption, thermal energy consumptions and moisture
content difference were calculated by the drying simulations when
the air input temperature was 38°C, the depth of the material bed
was 1 m, air velocity was 0.2 m/s, 0.3 m/s, 0.4 m/s, and 0.5 m/s,
and air direction switching time was 1 h, 2 h, 3 h, 4 h, and 5 h
(Figure 7). With the increase of air velocity, the time
consumption and moisture content difference decreased, and the
thermal energy consumed per unit mass increased significantly.
From 0.2 to 0.3 m/s, the moisture content difference decreased;
however, the decrease of moisture content difference was relatively
small when air velocity changed from 0.3 to 0.4 m/s and from
0.4 to 0.5 m/s. When the air direction switching time was 3 h, the
moisture content differences corresponding to air velocities of 0.3,
0.4, and 0.5 m/s were close.
a. Time consumption b. Moisture content difference c. Thermal energy consumption
Note: The ambient air temperature was 25°C, humidity was 50%, material bed depth was 1 m, and hot air input temperature was 38°C.
Figure 7 Drying indices at different air velocities and air direction switching times
In addition, when the air velocity remained constant, the air
direction switching time had little effect on the time consumption
and thermal energy consumed per unit mass; however, it had a
certain influence on the moisture content difference. An
appropriate air direction switching time was conducive to reducing
the moisture content difference. At an air velocity of 0.2 m/s, the
moisture content difference reached its minimum value when the
air direction switching time was 4 h; at an air velocity of 0.3 m/s,
the moisture content reached its minimum value when the air
direction switching time was 3 h; at an air ventilation velocity of
0.4 m/s, the moisture content difference reached its minimum value
when the air direction switching time was 2 h; and at an air velocity
of 0.5 m/s, the moisture content difference reached its minimum
value when the air direction switching time was 1 h. The larger
the air velocity, the smaller the best air direction switching time.
Combining time consumption, moisture content difference, and
thermal energy consumed per unit mass, when the ventilation
direction switching time was 3 h, the comprehensive operation
performance was better under different air velocities.
3.1.4 Stage division of segmented drying
Based on Figure 6, high temperature occurred at the later stage
of the drying process, thus maintaining the initial ventilation
velocity might cause a huge waste of energy. When the average
temperature is taken as the ordinate and the average moisture
content as the abscissa, a curve of temperature variation with
moisture content was obtained (Figure 8).
When the average moisture content was greater than 22%, the
average temperature fluctuated significantly with the average
moisture content. This stage occurred mainly during the rapid
heating at the beginning of the drying process. During the
following stage, the temperature of the material bed changed
steadily with decreasing average moisture content, and the
temperature drop amplitude caused by changing the ventilation
direction decreased gradually. Therefore, to reduce energy
consumption and improve heat utilization and uniformity of batch
drying, investigating segmented drying is considered indispensable.
The moisture content above 22% (wet basis) was taken as the first
stage. Meanwhile, the remaining drying stages were further
subdivided based on the variations of the average temperature.
Thus, moisture content of 14%-22% was taken as the second stage,
and a moisture content of 8%-14% was taken as the third stage.
Note: The ambient air temperature was 25°C and humidity was 50%; material
bed depth was 1 m, hot air input temperature was 38°C, and airflow reversing
time was 3 h.
Figure 8 Simulation results for variation of average temperature
with moisture content of the peanut bed
Four groups of ventilation velocities were then selected to
calculate the time consumption, moisture content difference, and
the thermal energy consumed per unit mass (Table 2). The results
showed that segmented drying could effectively reduce the
drying time and energy consumption and improve the drying
uniformity.
Table 2 Drying indices of segmented drying under different combinations of air velocities
No. Air velocity of different moisture content range/m·s
-1
Time consumption
/h
Moisture content
difference/%
Thermal energy consumption
/×106 J·kg
-1
40%-22% 22%-14% 14%-8%
1 0.3 0.3 0.3 46.9 0.58 3.56
2 0.3 0.3 0.2 47.9 0.56 3.12
3 0.4 0.3 0.3 43.7 0.54 3.59
4 0.4 0.3 0.2 44.7 0.56 3.17
September, 2021 Yan J C, et al. Optimizing the drying parameters of a fixed bed with reversing ventilation for peanut Vol. 14 No. 5 263
3.1.5 Uniform experimental design and simulation results
To obtain the optimal ventilation parameter combination of
segmented drying, referring to previous research experience, the
ventilation velocity of the three drying stages was set between 0.2
and 0.5 m/s. A U13*(134) uniform design table was used to
perform a 3-factor 13-level uniform design series of simulations
with the indicators of the time consumption, moisture content
difference, and thermal energy consumed per unit mass[45]. Based
on this, a synthetic weighted mark was calculated. The
mathematical simulation parameters and results are presented in
Table 3.
Table 3 U13*(134) Uniform design arrangement and simulation results
No.
Air velocity of different moisture content ranges/m·s-1